In general, a clutch apparatus mounted on an automobile, a motorcycle, or the like includes press-formed metal components each having a tubular boss portion and a flange portion extending from one end of the boss portion. Examples of these components include a clutch guide, an end plate, a clutch piston, a hub gear, and a pulley. In general, these press-formed components are formed by drawing, burring, punching, and cutting.
For example, below-listed Patent Document 1 discloses a method of forming a press-formed component. In the forming method, a preform is used. The preform is formed from a flat blank through drawing such that a protruding preform boss portion is formed at a center portion of the blank. After the wall thickness of the preform boss portion is increased, a projection end portion of the preform boss portion is removed, whereby the press-formed component is completed. In the method, the step of increasing the wall thickness of the preform boss portion is a so-called swaging process for increasing the wall thickness of the preform boss portion which has decreased as a result of the drawing of the preform boss portion. Specifically, the preform is disposed in a die apparatus in which flow of the material is restricted by an upper die, a lower die, and a lower punch. In this state, a corner portion of the preform boss portion is pressed from the outer side by an upper punch, whereby the wall thickness of the preform boss portion is increased.